Hydrostatic bearings are well known in the machine tool industry to provide the most accurate friction-free motion, and the longest life. With the need for ever greater accuracies to make higher quality parts, and the need for higher production rates which increase total accumulated travel on the machine bearings, an alternative is needed to the popular modular rolling element linear bearings that are commonly sold in the machine tool industry.
One feature of rolling element linear motion bearings is their relatively small profile. Another feature is their ease of use: A machine tool builder need only take the bearing out of the box and bolt it down. In today's world of modular order-what-you-want and it will be easily customized, it would be of great value to have a modular hydrostatic bearing that may be made bolt-for-bolt compatible with existing modular rolling element linear bearings.
There are a great many hydrostatic bearing designs currently available. All require a means to restrict the flow (compensation) as described, for ex ample, by A. H. Slocum in Precision Machine Design, Prentice Hall, Englewood Cliffs. N.J., 1992. Orifice compensated hydrostatic bearings, for example, are well-known in the art.
Unlike bearings that make mechanical contact, surface features cannot create error motions because the fluid layer always keeps components separated. Furthermore, unlike hydrodynamic bearings, the fluid film is kept at a nominal thickness by the external pressure source and the inlet restriction method that creates in effect a Wheatstone bridge fluid circuit. However, unlike a conventional bridge circuit, all the resistors are highly nonlinear, because they are in effect fluid resistors whose resistance is a function of flow between surfaces. Typically, the fluid resistance is a function of the bearing gap to the third power. However, the nominal bearing gaps are typically on the order of 20 microns, but manufacturing errors are on the order of 3 microns. Thus the nominal bearing resistance can vary from (20-3) 3=4913 to (20+3) 3=12167. This represents a threefold variance in initial conditions, whereas to obtain good performance, bridge resistors, which provide compensation to the circuit, should initially be within a few percent of each other. In the past, the inlet resistances have been made from orifices or capillaries, which are tuned to the bearing gaps on an individual basis. As a result, hydrostatic bearing machines are very expensive and have a reputation of being very fickle.
Applying hydrostatic bearing technology to the basic profile rail shape used for existing modular rolling element bearings, however, requires high pressures, in the range of 30 to 100 bar, in order to obtain reasonable load capacity and stiffness characteristics in view of the small surface areas available. The fact that the perimeter of each of the four hydrostatic bearing pockets required to support the bearing carriage is typically open to a drain to the atmosphere, furthermore, causes the flow rate to be very high. Reducing flow rate is highly desirable because it results in more efficient, lower cost filtration, and reduces drainage system cost. Furthermore, reducing flow rate reduces the pump power proportionally, which has major benefits including a lower cost and more compact pumping system, and less heat generation in the bearing fluid, reducing thermal errors.
To provide a durable means for preventing drainage from bearing pockets to the atmosphere, it is highly desirable to provide smooth contours, using large rounds between adjacent load bearing surfaces. In one prior art design with a profile rail, U.S. Pat. No. 4,978,233, all of the profile rails disclosed are formed with sharp edges. Sharp edges and/or very small rounds are, however, highly undesirable for sealing flow between adjacent bearing pockets because the fine edges are highly susceptible to damage and erosion, and also to manufacturing errors due to wear of fine edges on the forming tool. It should be noted, also, that in order to achieve hydrostatic stiffness, for each load bearing surface there must be at least one separate fluid pocket with its own individual pressure compensation means. In one embodiment of the prior art design of U.S. Pat. No. 4,978,233, separate load bearing pockets are not even provided, and hence individual compensation is impossible; and in the other embodiment presented, the compensation means presented using throttling holes is highly impractical because there is no means for tuning the resistances of the throttling holes once the disclosed bearing is assembled, and any such throttling holes shown are susceptible to deformation when the disclosed bearing insert is formed prior to assembly. The patentees, moreover, teach and show sharp-corner discontinuous design wherein a drain region is created between the pockets at the sharp rail corner, and they demonstrate no understanding whatsoever of the need, discovered by applicants herein, for smooth profile contours to prevent axial leakage between the pockets along the rail length, or of applicants discovery and teaching, later detailed, that a smooth blended profile contour between the load regions of the rail achieves a much greater degree of load capacity.
Improved operation is described in U.S. Pat. No. 5,104,237 by applicant Slocum wherein a method for using surface features on the bearings near the bearing pockets is employed to create the inlet fluid resistance. The pressure supply system fluid is therein flowed across a compensator land region to a collector groove which then connects to an opposing pocket by tubing or manifolding. Thus the compensator pocket is opposed to the bearing pocket, and a feedback circuit results.
In U.S. Pat. No. 5,466,071 by applicant Slocum and Wasson, there is disclosed the use of self compensation where the collected fluid is taken to the bearing pad by a thin channel that wraps around a shaft. Intuitively one would think that the fluid must be contained within a channel during its travel to the opposed bearing surface, but analysis showed this not to be true.
The problem with hydrostatic bearings, even those that are self-compensated as above, is that the bearings operate off of rectangular steel rails which are not compatible with existing popular profile bearing rails rolling element linear motion bearings. The pending invention in its revision, stemming from parent application Ser. No. 08/622,843 herein, provides an important solution through a design for a modular hydrostatic bearing that has a very high efficiency based on a special continuous smooth curve profile ground into the envelope of a profile rail bearing. This provides a durable means to cut the leakage flow in half, and enables easy manufacture of the bearing rail and mating carriage, such that orifice compensated, self-compensating, or other types of hydrostatic bearings can now easily be made modular and compatible with modular rolling element bearings admirably useful in linear motion bearing rail applications and the like.
The self-compensated form of the parent application presented herein requires that the fluid in the collector be manifolded to the opposed bearing pocket, while this is well deliverable, in the small spaces allotted to some modular hydrostatic bearings, this manifolding can be expensive. Also, it is difficult to avoid high leakage flows from the compensator supply groove where the small spaces are allotted.
In a similar philosophical approach to the surface compensation used by applicant Slocum and Wasson in said U.S. Pat. No. 5,416,071, the design of the present invention has been further modified in its special herein uses special shapes on the bearing surface to compensate the flow and deliver it directly to the bearing pocket without requiring any special tubing or manifold connections, thereby also enabling the invention to be more economically produced and operated even with small space bearing constructions.
Given these drawbacks, prior art hydrostatic modular linear motion bearings have not heretofore performed satisfactorily and thus have been deemed economically unfeasible.